Optical Image Stabilization (OIS) is a mechanism used in a still camera or video camera that stabilizes the recording of an individual image by varying the optical path to the sensor during image acquisition. This technology can be implemented within a lens or by moving the sensor as the final element in the optical path. The key element of all optical image stabilization systems is that they stabilize the image projected on the sensor before the sensor converts the corrected image into digital information.
Referring to FIG. 1, for example, Nikon and Canon's implementations of OIS work by using a floating lens element that is moved orthogonally to the optical axis of the lens using electromagnets, by a lens shift distance. Vibration is detected using two piezoelectric angular velocity sensors (often called gyroscopic sensors), not shown, one to detect horizontal movement and the other to detect vertical movement. When a camera is rotated as shown in FIG. 1, OIS compensates for the object motion caused by the camera rotation by shifting the lens barrel in order to maintain object position on the sensor. This type of compensation has its limitations: the optical image stabilizer corrects only for pitch and/or yaw axis rotations, and does not correct for rotation along the optical axis and as such, changes in perspective projection, caused by the camera rotation, are not compensated or perspective distortion can result from the correction applied; also the range of the correction provided by OIS is usually limited. Thus, as illustrated in the example of FIG. 2(a) where if a camera moves significantly during image acquisition, only a portion of this movement will be compensated leaving a distorted OIS corrected image. Note that the form of distorted image shown is caused by varying yaw movement during capture of an image using a rolling shutter.
Application of OIS is still popular due to fact that it can actively reduce the amount of motion blur in the final image due to fact that lens is following the motion of the image during image exposure time, keeping the center of the frame steady (within limits) on the sensor surface.
Electronic Image Stabilization (EIS) involves shifting images from frame to frame of video, enough to counteract inter-frame motion. EIS can be based only on frame-to-frame image analysis, using pixels outside the border of the visible frame to provide a buffer for motion. This technique reduces distracting vibrations within videos by smoothing the transition from one frame to another. This technique does not affect the noise level of the image, except in the extreme borders if the image is extrapolated. It cannot do anything about existing motion blur, which may result in an image seemingly losing focus as motion is compensated.
However, this approach has its own problems as it can become unreliable in certain situations (lack of details, large moving objects, and repetitive patterns).
Some forms of EIS are supplemented with measurements of camera motion provided by camera inertial sensors (IMU). In that case, knowing the camera intrinsic properties, the motion in the sensor plane can be recreated and filtered in order to provide a stable video sequence, albeit with potentially camera motion blurred images.
Note that OIS and EIS stabilizations, especially those based on IMU external to the lens, are not used together. This is because OIS introduces image motion that no longer correlates with the camera motion. For example, let us assume that OIS is correcting 20% of camera shake. An IMU-based EIS stabilization would have no information about this correction and would attempt to compensate for 100% of the camera shake. This results in overcorrection that is visible in the resulting video as a residual camera shake. This shake is particularly visible as it does not follow natural motion patterns. This situation is illustrated by FIG. 2(b) which shows the correction applied according to camera trajectory on an image that was already partially corrected by OIS. Instead of straight objects, we get objects bent in the opposite direction.